A simple model potential for the electron-Ca-atom interaction is used to describe the process of charge transfer in Rydberg-atom-calcium-atom collisions. The interaction of the final ionic state with degenerate manifolds of Rydberg covalent states is treated by inclusion of the negative ion decay due to the Coulomb field of the positive ion. This effect leads to a strong reduction of the charge transfer cross section and shifting of the value of the principal quantum number n corresponding to the cross section maximum towards higher n. The rates averaged over the experimental velocity distribution agree quite well with the experimental data and with independent calculations using the Landau-Zener model, although the results of the latter are higher than the present for the charge transfer from s states. The distortion of the classical ion trajectories due to the Coulomb interaction plays a minor role in the calculation of the negative-ion decay probability.